Having worked as a frontend dev for a while, my observation is this - because the UI is closer to the user, and closer to the product side (for consumer-facing apps), it has faster iteration cycles, doubling or tripling the rate of technology decisions that are made.

You'd never rewrite an API twice in a year, but maybe you'd rebuild your site or interface that often, if you're responding to users.

With increased demand for tools comes more supply - more libraries and frameworks with more stars and follows.

The reason we don't see this in the mobile world is because it's a much more closed ecosystem - there's no open spec for mobile like there is for browsers.